The last list is of online FPS demos that still have servers with people playing. All of these demos are worth trying because they are different from regular free online FPS games. Almost all demos listed here have vehicles in-game, which is not very common in regular free-to-play shooters. The downside is that multiplayer demos usually have only one map and couple game modes, so it might get boring after a while.
Battlefield 2 is a modern warfare first-person shooter. This demo includes the "Gulf of Oman" map in many versions, and lots of vehicles: tanks, APCs, aircrafts, helicopters, boats and cars.System requirements: 1.7 Ghz CPU, 512 MB RAM, 128 MB NVIDIA FX 5700.
Halo: Combat Evolved
Halo: Combat Evolved is a Sci-Fi first-person shooter. This FPS demo includes one single-player mission and one multiplayer map, "Blood Gulch" with five play modes.System requirements: 733 MHz CPU, 128MB RAM, 32 MB T&L capable video card.
Enemy Territory: Quake Wars
Enemy Territory: Quake Wars is a Sci-Fi first-person shooter. The demo contains a single map, "Valley" and 5 classes playable as a human - Earth Defense Force or an alien - Strogg.System requirements: 2.8 GHz Pentium 4, 512MB RAM, 128MB NVIDIA FX 5700.
Unreal Tournament 2004
Unreal Tournament 2004 is a futuristic first-person shooter with various aircrafts and land vehicles. This multiplayer demo contains 7 different maps and 7 game modes.Minimum requirements: 1.0 GHz CPU, 128MB RAM, DirectX 9.0b 32 MB graphics card.
Download (also available for Linux and Mac)
Battlefield 1942 is a World War II first-person shooter. This multiplayer demo includes the "Wake Island" map with many vehicles (planes, tanks, cars, ships etc.).System requirements: 500 MHz CPU, 128 MB RAM, 3D accelerated 32 MB video card.
Battlefield 1942: Secret Weapons of WWII
Battlefield 1942: Secret Weapons of WWII is an expansion pack to Battlefield 1942. The multiplayer demo contains one map, "Hellendoorn" and many new vehicles.System requirements: 500 MHz CPU, 128 MB RAM, 3D accelerated 32 MB video card.
Soldier of Fortune 2
Soldier of Fortune II is a tactical first-person shooter. SoF 2 demo contains a single map and 4 different play modes: deathmatch, team deathmatch, elimination and infiltration.System requirements: 450 MHz CPU, 128MB RAM, 16 MB DirectX-compatible video card.